This is actually a good argument for a California/Washington style system for mayoral elections in cities where the Democratic primary is all that matters. It sucks for statewide elections but it is more representative and democratic in one-party municipalities. Minneapolis basically used such a system before switching to IRV. Fenty probably would've won a basically non-partisan open to all voters runoff.
Would be better to just make elections non-partisan with a top-two run-off.
Then we'd have a more difficult time figuring out who the Republicans are. Better to keep party names on the ballot.